TableGen: Reimplement !foreach using the resolving mechanism

Summary:
This changes the syntax of !foreach so that the first "parameter" is
a new syntactic variable: !foreach(x, lst, expr) will define the
variable x within the scope of expr, and evaluation of the !foreach
will substitute elements of the given list (or dag) for x in expr.

Aside from leading to a nicer syntax, this allows more complex
expressions where x is deeply nested, or even constant expressions
in which x does not occur at all.

!foreach is currently not actually used anywhere in trunk, but I
plan to use it in the AMDGPU backend. If out-of-tree targets are
using it, they can adjust to the new syntax very easily.

Change-Id: Ib966694d8ab6542279d6bc358b6f4d767945a805

Reviewers: arsenm, craig.topper, tra, MartinO

Subscribers: wdng, llvm-commits, tpr

Differential Revision: https://reviews.llvm.org/D43651

llvm-svn: 326705